The present invention relates to circuit arrangements for energizing the electromotor driving a blower in a commercial vehicle. In particular it relates to electromotors driving blowers having a drum rotor with vanes which are curved in the forward direction. Such blowers are used in air conditioning systems in commercial vehicles.
It is known that voltage controlled motors, that is for example a permanent magnet motor which is connected to a voltage source through possibly a variable series resistance, will draw more current for increasing loads, the speed of the motor decreasing simultaneously with the increase in current. If the motor blocks, then, in spite of the relatively low voltage applied to the motor, a sufficiently high current, limited only by the shortcircuit resistance of the motor coils, flows through the motor causing the motor to overheat and an overload to be applied to the whole supply system.
Voltage control of the motor is particularly disadvantageous when the electromotor drives a blower which is subject to large load variations. This effect occurs particularly in air conditioner blowers which are mounted in commercial vehicles. The power drawn by the motor then varies in strong dependence upon the speed of the vehicle or, in other words, on the pressure head produced by the speed. For increasing speeds and thus increasing pressure heads, the electromotor often draws so high a current that either the motor is burned out or the battery voltage collapses. This effect of increasing current consumption takes place particularly when the electromotor drives a radial type blower which has a drum rotor with vanes which are curved in the forward direction.